Display device and driving method of the same

ABSTRACT

A problem in that a light emitting element slightly emits light is solved by an off current of a thin film transistor connected in series to the light emitting element, thereby a display device which can perform a clear display by increasing contrast, and a driving method thereof are provided. When the thin film transistor connected in series to the light emitting element is turned off, a charge held in the capacitance of the light emitting element itself is discharged. Even when an off current is generated at the thin film transistor connected in series to the light emitting element, this off current charges this capacitance until the capacitance of the light emitting element itself holds a predetermined voltage again. Accordingly, the off current of the thin film transistor does not contribute to light emission. In this manner, a slight light emission of the light emitting element can be reduced.

This application is a continuation of U.S. application Ser. No.11/222,152 filed on Sep. 8, 2005 now U.S. Pat. No. 8,044,895.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device of an active matrixdriving method in which a switching element is provided in each pixel,and a driving method thereof. In particular, the invention relates to adisplay device in which a luminance of a light emitting element iscontrolled by controlling a current supplied to the light emittingelement by using a switching element connected in series to the lightemitting element, and a driving method thereof. More particularly, theinvention relates to a display device including an element with diodecharacteristics as a light emitting element and a driving methodthereof.

2. Description of the Related Art

A display device including a thin film transistor as a switching elementand a driving method thereof are suggested. FIG. 8A shows an example ofa pixel configuration thereof.

In FIG. 8A, reference numeral 105 denotes a light emitting element, 102denotes a thin film transistor, 103 denotes a first power source line,and 104 denotes a second power source line. The light emitting element105 includes two electrodes and emits light at a luminance according toa supplied current value of a current flowing between two electrodes.One of the two electrodes of the light emitting element 105 is referredto as a first electrode 105 a while the other is referred to as a secondelectrode 105 b. In the pixel shown in FIG. 8A, a value of a current(hereinafter referred to as a drain current) flowing between a sourceand drain of the thin film transistor 102 is controlled according to apotential G1 applied to a gate of the thin film transistor 102. Thedrain current of the thin film transistor 102 flows between the firstelectrode 105 a and the second electrode 105 b of the light emittingelement 105 connected in series to the thin film transistor 102. Thelight emitting element emits light at a luminance according to asupplied current. In this manner, by controlling the drain current ofthe thin film transistor 102, the luminance of the light emittingelement 105 is controlled to perform a display.

An electroluminescence element and the like can be used as the lightemitting element 105. An electroluminescence element has diodecharacteristics to flow a current in only one direction. FIG. 8B showsthe light emitting element 105 in FIG. 8A as a diode. In FIG. 8B, thefirst electrode 105 a is an anode and the second electrode 105 b is acathode.

A display device in which a reverse bias voltage is regularly applied tothe light emitting element 105 as well as a forward bias voltage isapplied to the light emitting element 105 to emit light, and a drivingmethod thereof have been suggested (see Patent Document 1).

Patent Document 1

-   Japanese Patent Laid-Open No. 2002-190390

SUMMARY OF THE INVENTION

Hereinafter considered is the case where the light emitting element 105emits no light so as to display “black” in FIG. 8B. A potential G1 isset appropriately to set a potential difference between the source andgate of the thin film transistor 102 to be equal to or lower than athreshold voltage of the thin film transistor 102 or lower, thereby thethin film transistor 102 is turned off. In this manner, the draincurrent of the thin film transistor 102 becomes zero so that the lightemitting element 105 emits no light to display “black”. It is preferablethat the thin film transistor 102 be completely turned off when avoltage equal to or lower than the threshold voltage is applied to thesource and gate, however, the thin film transistor 102 is not completelyturned off in actuality and a slight amount of drain current flows. Thiscurrent is denoted as I_(off) in the drawing and referred to as an offcurrent. Due to the off current I_(off), the light emitting elementwhich is not supposed to emit light emits light (hereinafter thisphenomenon is referred to as grayish black effect). Accordingly, thereis a problem in that contrast of a display is decreased.

In particular, in the case where the light emitting element 105continues to operate with a forward bias voltage, that is the case wherethe light emitting element 105 continues to operate with a potential ofthe first electrode 105 a (anode) being higher than that of the secondelectrode 105 b (cathode), such a problem is found that a grayish blackeffect becomes remarkable.

It is found out that the grayish black effect becomes remarkable in thecase where the light emitting element 105 continues to operate with aforward bias voltage because a voltage approximately the same as thethreshold voltage of the light emitting element 105 is constantly heldbetween the first electrode 105 a and the second electrode 105 b.

The threshold voltage of the light emitting element is V_(th) in FIG.8C. FIG. 8C shows a relationship of a current I flowing from the anodeto cathode relatively to a potential difference V_(EL) between apotential at the second electrode 105 b (cathode) and a potential at thefirst electrode 105 a (anode). When V_(EL) becomes higher than thethreshold voltage V_(th), the current I flows. That is, a current flowsin the light emitting element 105 when a voltage higher than thethreshold voltage V_(th) is applied between the first electrode 105 a(anode) and the second electrode 105 b (cathode), thereby the lightemitting element 105 emits light.

A voltage approximately the same as the threshold voltage V_(th) is heldbetween the first electrode 105 a (anode) and the second electrode 105 b(cathode) of the light emitting element 105 because of capacitance ofthe light emitting element 105 itself. FIG. 8D shows a light emittingelement shown as a diode and FIG. 8E shows an equivalent circuit diagramof FIG. 8D. A capacitor 800 in the equivalent circuit corresponds tocapacitance of the light emitting element 105 itself. The thresholdvoltage V_(th) is held by the capacitor 800.

In the case where the light emitting element 105 continues to operatewith a forward bias voltage, a potential of the first electrode 105 a(anode) continues to be higher than that of the second electrode 105 b(cathode) even after the thin film transistor 102 is turned off, therebya voltage approximately the same as the threshold voltage V_(th) is heldin the capacitor 800 of the light emitting element 105. Therefore, whenan off current I_(off) is generated in the thin film transistor 102, theoff current I_(off) flows to a path 801 a on a diode 802 side withoutflowing to a path 801 b on the capacitor 800 side in the equivalentcircuit of FIG. 8E, thereby contributing to light emission. In thismanner, the inventors found out that the grayish black effect becomesremarkable in the case where the light emitting element 105 continues tooperate with a forward bias voltage.

The invention provides a display device which can perform a cleardisplay by reducing grayish black effect and increasing contrast, and adriving method thereof.

In order to reduce the grayish black effect, the display device anddriving method of the invention employ a following first configurationor second configuration.

[First Configuration]

In the case where a first electrode of a light emitting element is ananode and a second electrode thereof is a cathode, a potential of asecond power source line is set so that a potential of the firstelectrode becomes equal to or higher than that of the second electrodeand that a voltage applied between the first electrode and the secondelectrode becomes smaller than a threshold voltage of the light emittingelement when a first thin film transistor connected in series to thelight emitting element is selected to be turned off.

In the case where a first electrode of a light emitting element is acathode and a second electrode thereof is an anode, a potential of asecond power source line is set so that a potential of the firstelectrode becomes equal to or lower than that of the second electrodeand that a voltage applied between the first electrode and the secondelectrode becomes smaller than a threshold voltage of the light emittingelement when a first thin film transistor connected in series to thelight emitting element is selected to be turned off.

[Second Configuration]

In the case where a first electrode of a light emitting element is ananode and a second electrode thereof is a cathode, a second thin filmtransistor is provided which is different than a first thin filmtransistor connected in series to the light emitting element. One of asource and drain of the second thin film transistor is connected to thefirst electrode of the light emitting element and the other is connectedto a power source line. When the first thin film transistor is selectedto be turned off, the second thin film transistor is selected to beturned on and a potential of the power source line is set equal to orhigher than a potential of the second electrode of the light emittingelement and lower than a potential obtained by adding the potential ofthe second electrode to a threshold voltage of the light emittingelement.

In the case where the first electrode of the light emitting element is acathode and the second electrode thereof is an anode, a second thin filmtransistor is provided which is different than the first thin filmtransistor connected in series to the light emitting element. One of asource and drain of the second thin film transistor is connected to thefirst electrode of the light emitting element and the other is connectedto the power source line. When the first thin film transistor isselected to be turned off, the second thin film transistor is selectedto be turned on and a potential of the power source line is set equal toor lower than a potential of the second electrode of the light emittingelement and higher than a potential obtained by subtracting a thresholdvoltage of the light emitting element from the potential of the secondelectrode.

It is to be noted that the power source line connected to the secondthin film transistor can be shared as a power source line connected tothe second electrode of the light emitting element.

According to the first and second configuration, a thin film transistorhaving an active layer formed of a polycrystalline semiconductor can beused as the first thin film transistor.

According to the first and second configuration, a third configurationdescribed next can be used in combination.

[Third Configuration]

A capacitor is provided to be connected in parallel to a light emittingelement.

That is, the capacitor is provided so that one electrode is connected toa first electrode of the light emitting element and the other electrodeis connected to a second electrode of the light emitting element.

According to a display device and a driving method thereof of theinvention, when a thin film transistor connected in series to a lightemitting element is selected to be turned off so that a light emittingelement emits no light, a charge corresponding to a threshold voltageheld in the capacitance of the light emitting element itself can bedischarged. Accordingly, when an off current is generated in the thinfilm transistor connected in series to the light emitting element, theoff current flows to charge the capacitance of the light emittingelement itself until the capacitance of the light emitting elementitself holds a threshold voltage again. Therefore, the off current ofthe thin film transistor does not contribute to light emission for awhile after the thin film transistor connected in series to the lightemitting element is selected to be turned off. In this manner, a grayishblack effect can be reduced. Accordingly, according to the displaydevice and the driving method thereof of the invention, a clear displaycan be performed by increasing the contrast of the display.

According to the first and second configurations, when the thin filmtransistor connected in series to the light emitting element is selectedto be turned off so that the light emitting element emits no light, aforward bias voltage is applied between the electrodes of the lightemitting element and the voltage is set lower than the threshold voltageof the light emitting element. According to both of the first and secondconfigurations, a reverse bias voltage is not applied to the lightemitting element. Accordingly, compared to a method for regularlyapplying a reverse bias voltage to the light emitting element, powerconsumption can be reduced according to the display device and thedriving method thereof of the invention.

According to the second configuration, the power source line connectedto the second thin film transistor is shared as a power source lineconnected to the second electrode of the light emitting element, therebythe number of wiring lines can be reduced and an aperture ratio of pixelcan be improved.

Compared to a thin film transistor having an active layer formed of asingle crystalline semiconductor or an amorphous semiconductor, a thinfilm transistor having an active layer formed of a polycrystallinesemiconductor produces more off current due to a crystal grain boundaryand the like. Therefore, the invention is efficient particularly in thecase of using a thin film transistor having an active layer formed of apolycrystalline semiconductor as a first thin film transistor.

By using the first and second configurations in combination, an offcurrent of the thin film transistor connected in series to the lightemitting element continues to flow to the capacitor until it is charged.Therefore, longer time can be taken after selecting to turn off the thinfilm transistor connected in series to the light emitting element untilthe off current of the thin film transistor starts to contribute to thelight emission. In this manner, a grayish black effect can further bereduced.

As described above, the invention provides a display device which canperform a clear display with a higher contrast and less powerconsumption, and a driving method thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are diagrams showing Embodiment Mode 1 of the invention.

FIGS. 2A to 2C are diagrams showing Embodiment Mode 2 of the invention.

FIGS. 3A to 3D are diagrams showing Embodiment Mode 3 of the invention.

FIGS. 4A to 4D are diagrams showing Embodiment Mode 4 of the invention.

FIGS. 5A to 5D are diagrams showing Embodiment 1 of the invention.

FIGS. 6A and 6B are diagrams showing Embodiment 2 of the invention.

FIGS. 7A and 7B are diagrams showing Embodiment 3 of the invention.

FIGS. 8A and 8B show conventional configurations and FIGS. 8C to 8E showconfigurations of a light emitting element.

FIGS. 9A to 9D are diagrams showing Embodiment 10 of the invention.

FIG. 10 is a diagram showing Embodiment 4.

FIG. 11 is a diagram showing Embodiment 5.

FIGS. 12A to 12C are diagrams showing Embodiment 6.

FIGS. 13A to 13C are diagrams showing Embodiment 7.

FIGS. 14A to 14C are diagrams showing Embodiment 8.

FIGS. 15A to 15C are diagrams showing Embodiment 9.

DETAILED DESCRIPTION OF THE INVENTION

Although the present invention will be fully described by way ofEmbodiment Modes and Embodiments with reference to the accompanyingdrawings, it is to be understood that various changes and modificationswill be apparent to those skilled in the art. Therefore, unlessotherwise such changes and modifications depart from the scope of theinvention, they should be construed as being included therein. Note thatidentical portions in embodiment modes and embodiments are denoted bythe same reference numerals and detailed descriptions thereof areomitted.

Embodiment Mode 1

An example in which the first and third configurations are used incombination is described with reference to FIGS. 1A to 1C. In FIGS. 1Ato 1C, reference numeral 105 denotes a light emitting element, 102denotes a thin film transistor, 103 denotes a first power source line,104 denotes a second power source line, 101 denotes a capacitor, and 106denotes a circuit for setting a potential. The circuit 106 for setting apotential corresponds to the first configuration. The capacitor 101corresponds to the third configuration. The light emitting element 105has two electrodes and emits light at a luminance according to a currentvalue of a current flowing between the two electrodes. One of the twoelectrodes of the light emitting element 105 is referred to as a firstelectrode 105 a while the other is referred to as a second electrode 105b.

A method for using the first and third configurations in combination isdescribed with reference to FIG. 1A.

When the thin film transistor 102 connected in series to the lightemitting element 105 is selected to be turned off, the circuit 106 forsetting a potential changes a potential of the second power source line104 so that a forward bias voltage is applied between electrodes of thelight emitting element 105 and the voltage becomes lower than thethreshold voltage of the light emitting element 105. In this manner, acharge held in the capacitance of the light emitting element 105 itselfis discharged to reduce slight light emission of the light emittingelement 105.

When the thin film transistor 102 connected in series to the lightemitting element 105 is selected to be turned on, the circuit 106 forsetting a potential changes a potential of the second power source line104 so that a forward bias voltage is applied between electrodes of thelight emitting element 105 and the voltage becomes higher than thethreshold voltage of the light emitting element 105. A drain current ofthe thin film transistor 102 which flows according to a potential G1applied to a gate thereof flows to the light emitting element 105. Thelight emitting element 105 emits light at a luminance according to thedrain current. In this manner, the luminance of the light emittingelement 105 is controlled to perform a display.

The circuit 106 for setting a potential can have, for example, aconfiguration including a switch and two terminals (referred to as afirst terminal and a second terminal) which are applied differentpotentials. The switch selects a connection between the first terminaland the second power source line 104 or a connection between the secondterminal and the second power source line 104. When the thin filmtransistor 102 is selected to be turned off, the switch connects thefirst terminal and the second power source line 104 while the switchconnects the second terminal and the second power source line 104 whenthe thin film transistor 102 is selected to be turned on. The firstterminal is applied such a voltage as to apply a forward bias voltagebetween the electrodes of the light emitting element 105 in relation toa potential applied to the first power source line 103 and set theapplied voltage lower than the threshold voltage of the light emittingelement 105. The second terminal is applied such a voltage as to apply aforward bias voltage between the electrodes of the light emittingelement 105 in relation to a potential applied to the first power sourceline 103 and set the applied voltage higher than the threshold voltageof the light emitting element 105.

A gray scale can be displayed by controlling the time in which the lightemitting element 105 emits light in one frame period.

One electrode of the capacitor 101 is connected to the first electrode105 a while the other electrode is connected to the second electrode 105b. That is, the capacitor 101 is connected in parallel to the lightemitting element 105. An off current of the thin film transistor 102connected in series to the light emitting element 105 flows to thecapacitor 101 provided additionally until it is charged. Therefore,longer time can be taken after turning off the thin film transistor 102connected in series to the light emitting element 105 until the offcurrent of the thin film transistor 102 starts to contribute to thelight emission. In this manner, a grayish black effect can further bereduced.

An electroluminescence element and the like can be used as the lightemitting element 105. The electroluminescence element has diodecharacteristics to flow a current in only one direction. Each of FIGS.1B and 1C shows the light emitting element 105 in FIG. 1A as a diode. InFIG. 1B, the first electrode 105 a is an anode and the second electrode105 b is a cathode. In FIG. 1C, the first electrode 105 a is a cathodeand the second electrode 105 b is an anode.

The circuit 106 for setting a potential in FIG. 1B is described.

When the thin film transistor 102 is selected to be turned on, apotential of the second power source line 104 is set so that a potentialof the first electrode 105 a becomes higher than that of the secondelectrode 105 b and a voltage applied between the first electrode 105 aand the second electrode 105 b becomes higher than the threshold voltageof the light emitting element 105, thereby the light emitting element105 emits light.

When the thin film transistor 102 is selected to be turned off, apotential of the second power source line 104 is set so that a potentialof the first electrode 105 a becomes equal to or higher than that of thesecond electrode 105 b and a voltage applied between the first electrode105 a and the second electrode 105 b becomes lower than the thresholdvoltage of the light emitting element 105, thereby the light emittingelement 105 emits no light.

The circuit 106 for setting a potential in FIG. 1C is described.

When the thin film transistor 102 is selected to be turned on, apotential of the second power source line 104 is set so that a potentialof the first electrode 105 a becomes lower than that of the secondelectrode 105 b and a voltage applied between the first electrode 105 aand the second electrode 105 b becomes higher than the threshold voltageof the light emitting element 105, thereby the light emitting element105 emits light.

When the thin film transistor 102 is selected to be turned off, apotential of the second power source line 104 is set so that a potentialof the first electrode 105 a becomes equal to or lower than that of thesecond electrode 105 b and a voltage applied between the first electrode105 a and the second electrode 105 b is lower than the threshold voltageof the light emitting element 105, thereby the light emitting element105 emits no light.

Embodiment Mode 2

An example of using the second and third configurations in combinationis described with reference to FIGS. 2A to 2C. In FIGS. 2A to 2C, thesame reference numerals are used for the same portions as in FIGS. 1A to1C, and a description thereon is omitted. Reference numeral 107 denotesa thin film transistor. The thin film transistor 107 and a third powersource line 204 correspond to the second configuration. The capacitor101 corresponds to the third configuration.

A method for using the second and third configurations in combination isdescribed in details with reference to FIG. 2A.

When the thin film transistor 102 connected in series to the lightemitting element 105 is selected to be turned off, the thin filmtransistor 107 is selected to be turned on by controlling a potential G2applied to a gate thereof. In this manner, a potential of the thirdpower source line 204 is applied to the first electrode 105 a. When thethin film transistor 107 is selected to be turned on, a potentialdifference between the second power source line 104 and the third powersource line 204 is set zero or higher and lower than the thresholdvoltage of the light emitting element 105. In this manner, a charge heldin the capacitance of the light emitting element 105 itself isdischarged to reduce slight light emission of the light emitting element105.

The second power source line 104 and the third power source line 204 canbe shared as well. In this manner, an aperture ratio of pixel can beimproved by reducing the number of wirings.

The thin film transistor 107 is selected to be turned off by controllingthe potential G2 applied to a gate when the thin film transistor 102connected in series to the light emitting element 105 is selected to beturned on. A drain current of the thin film transistor 102 which flowsaccording to the potential G1 applied to the gate thereof flows to thelight emitting element 105. The light emitting element 105 emits lightat a luminance according to the drain current. In this manner, theluminance of the light emitting element 105 is controlled to perform adisplay.

A gray scale can be displayed by controlling the time in which the lightemitting element 105 emits light in one frame period.

By providing the capacitor 101, longer time can be taken after selectingto turn off the thin film transistor 102 connected in series to thelight emitting element 105 until the off current of the thin filmtransistor 102 starts to contribute to the light emission. In thismanner, a grayish black effect can further be reduced.

An electroluminescence element and the like can be used as the lightemitting element 105. The electroluminescence element has diodecharacteristics to flow a current in only one direction. Each of FIGS.2B and 2C shows the light emitting element 105 in FIG. 2A as a diode. InFIG. 2B, the first electrode 105 a is an anode and the second electrode105 b is a cathode. In FIG. 2C, the first electrode 105 a is a cathodeand the second electrode 105 b is an anode.

Embodiment Mode 3

An example of using the first and third configurations in combination isdescribed with reference to FIGS. 3A to 3D. In FIGS. 3A to 3D, the samereference numerals are used for the same portions as in FIGS. 1A to 1Cand a description thereon is omitted.

Each of FIGS. 3A and 3B corresponds to FIG. 1A provided with thecapacitor 108. The capacitor 108 is provided to hold a gate-sourcevoltage of the thin film transistor 102. FIG. 3A shows an example wherethe thin film transistor 102 has a source on a side connected to thefirst power source line 103. FIG. 3B shows an example where the thinfilm transistor 102 has a source on a side connected to the firstelectrode 105 a of the light emitting element 105.

A drain current of the thin film transistor 102 changes according to apotential difference between a source potential thereof and the gatepotential G1. Even if the gate potential G1 of the thin film transistor102 is controlled, when a source potential changes, a potentialdifference between the source potential and the gate potential changes,which changes the drain current. Accordingly, it is preferable to keepthe source potential of the thin film transistor 102 constant.Therefore, it is preferable that the thin film transistor 102 have asource on a side connected to the first power source line 103 as shownin FIG. 3A.

Each of FIGS. 3C and 3D corresponds to FIG. 3A in which the lightemitting element 105 is shown as a diode. The first electrode 105 a isan anode and the second electrode 105 b is a cathode in FIG. 3C. Thefirst electrode 105 a is a cathode and the second electrode 105 b is ananode in FIG. 3D.

In FIG. 3C, a current flows from the first power source line 103 in adirection to the second power source line 104, thereby the lightemitting element 105 emits light. A potential of the thin filmtransistor 102 on a side connected to the first power source line 103becomes higher than that on a side connected to the first electrode 105a of the light emitting element 105. A p-channel transistor is used asthe thin film transistor 102 so that the thin film transistor 102 has asource on the side connected to the first power source line 103.

In FIG. 3D, a current flows from the second power source line 104 in adirection to the first power source line 103, thereby the light emittingelement 105 emits light. A potential of the thin film transistor 102 onthe side connected to the first electrode 105 a of the light emittingelement 105 becomes higher than that on the side connected to the firstpower source line 103. An n-channel transistor is used as the thin filmtransistor 102 so that the thin film transistor 102 has a source on theside connected to the first power source line 103.

Embodiment Mode 4

An example of using the second and third configurations in combinationis described with reference to FIGS. 4A to 4D. In FIGS. 4A to 4D, thesame reference numerals are used for the same portions as in FIGS. 2A to3D.

Each of FIGS. 4A and 4B corresponds to FIG. 2A provided with thecapacitor 108. The capacitor 108 is provided to hold a gate-sourcevoltage of the thin film transistor 102. FIG. 4A shows an example wherethe thin film transistor 102 has a source on a side connected to thefirst power source line 103. FIG. 4B shows an example where the thinfilm transistor 102 has a source on a side connected to the firstelectrode 105 a of the light emitting element 105.

FIG. 4A is preferable in that the thin film transistor 102 has a sourceon the side connected to the first power source line 103 similarly toFIG. 3A.

Each of FIGS. 4C and 4D corresponds to FIG. 4A in which the lightemitting element 105 is shown as a diode. The first electrode 105 a isan anode and the second electrode 105 is a cathode in FIG. 4C. The firstelectrode 105 a is a cathode and the second electrode 105 b is an anodein FIG. 4D.

In FIG. 4C, a p-channel transistor is used as the thin film transistor102 so that the thin film transistor 102 has a source on the sideconnected to the first power source line 103. In FIG. 4D, an n-channeltransistor is used as the thin film transistor 102 so that the thin filmtransistor 102 has a source on the side connected to the first powersource line 103.

Embodiment 1

A specific example of a pixel using a configuration described inembodiment modes is described with reference to FIGS. 5A to 5D. In FIGS.5A to 5D, the same reference numerals are used for the same portions asin FIGS. 1A to 4D, and a description thereon is omitted.

FIG. 5A shows the configuration of FIG. 1A showing a specific example ofa circuit for inputting the potential G1 to the gate of the thin filmtransistor 102. FIG. 5B shows the configuration of FIG. 2A showing aspecific example of a circuit for inputting the potential G1 to the gateof the thin film transistor 102. FIG. 5C shows the configuration of FIG.3A showing a specific example of a circuit for inputting the potentialG1 to the gate of the thin film transistor 102. FIG. 5D shows theconfiguration of FIG. 4A showing a specific example of a circuit forinputting the potential G1 to the gate of the thin film transistor 102.

In FIGS. 5A to 5D, reference numeral 500 denotes a pixel, 501 denotes athin film transistor, 502 denotes a signal line, and 503 denotes a scanline. One of a source and drain of the thin film transistor 501 isconnected to the signal line 502 while the other is connected to thegate of the thin film transistor 102. A gate of the thin film transistor501 is connected to the scan line 503.

In the configurations shown in FIGS. 5A to 5D, when the thin filmtransistor 501 is selected to be turned on by a signal inputted to thescan line 503, a signal inputted to the signal line 502 is inputted tothe gate of the thin film transistor 102. In this manner, on/off of thethin film transistor 102 and a value of a drain current thereof whenturned on are controlled.

In FIG. 5A, an operation of the circuit 106 for setting a potential ineach of the case where the thin film transistor 102 is selected to beturned on and the case where the thin film transistor 102 is selected tobe turned off is similar to Embodiment Mode 1. In FIG. 5B, an operationof the thin film transistor 107 in each of the case where the thin filmtransistor 102 is turned on and the case where the thin film transistor102 is selected to be turned off is similar to Embodiment Mode 2. InFIG. 5C, an operation of the circuit 106 for setting a potential in eachof the case where the thin film transistor 102 is selected to be turnedon and the case where the thin film transistor 102 is selected to beturned off is similar to Embodiment Mode 3. In FIG. 5D, an operation ofthe thin film transistor 107 in each of the case where the thin filmtransistor 102 is selected to be turned on and the case where the thinfilm transistor 102 is selected to be turned off is similar toEmbodiment Mode 4.

Embodiment 1 can be freely implemented in combination with embodimentmodes.

Embodiment 2

A different example than the example of the pixel shown in Embodiment 1is described with reference to FIGS. 6A and 6B. In FIGS. 6A and 6B, thesame reference numerals are used for the same portions as in FIGS. 1A to5D, and a description thereon is omitted.

FIG. 6A shows the configuration of FIG. 5C in which a circuit forselecting to turn off the thin film transistor 102 independent of asignal of the signal line 502 is provided. FIG. 6B shows theconfiguration of FIG. 5D in which a circuit for selecting to turn offthe thin film transistor 102 independent of a signal of the signal line502 is provided.

In FIGS. 6A and 6B, reference numeral 601 denotes a thin filmtransistor. One of a source and drain of the thin film transistor 601 isconnected to one electrode of the capacitor 108 and the other isconnected to the other electrode of the capacitor 108.

In the configurations shown in FIGS. 6A and 6B, the thin film transistor601 is selected to be turned on by a potential G3 inputted to a gatethereof so that two electrodes of the capacitor 108 have approximatelythe same potentials. The charge held in the capacitor 108 is discharged,thereby a potential difference between the source and gate of the thinfilm transistor 102 becomes approximately zero. In this manner, the thinfilm transistor 102 is selected to be turned off.

According to the second configuration, when the thin film transistor 102connected in series to the light emitting element 105 is selected to beturned off, the thin film transistor 107 is selected to be turned on.Accordingly, a timing to select to turn on the thin film transistor 601and a timing to select to turn on the thin film transistor 107 in FIG.6B can be the same. Therefore, the thin film transistors 107 and 601 canhave the same polarity and gates thereof can be connected to the samewiring, thereby signals can be inputted thereto at the same time. Awiring for inputting a signal to the gate of the thin film transistor107 and a wiring for inputting a signal to the gate of the thin filmtransistor 601 can be shared, which can improve an aperture ratio ofpixels.

This embodiment mode can be freely implemented in combination withembodiment modes.

Embodiment 3

Another example than the examples of the pixel described in Embodiments1 and 2 is described with reference to FIGS. 7A and 7B. In FIGS. 7A and7B, the same reference numerals are used for the same portions as inFIGS. 1A to 6B.

FIG. 7A shows a specific example of a circuit for inputting thepotential G1 to the gate of the thin film transistor 102 in theconfiguration of FIG. 3A. FIG. 7B shows a specific example of a circuitfor inputting the potential G1 to the gate of the thin film transistor102 in the configuration of FIG. 4A.

In FIGS. 7A and 7B, reference numerals 701, 702, and 703 denote thinfilm transistors, 704 denotes a signal line, and 705 denotes a scanline. One of a source and drain of the thin film transistor 702 isconnected to the signal line 704 and the other is connected to one of asource and drain of the thin film transistor 701 and one of a source anddrain of the thin film transistor 703. A gate of the thin filmtransistor 702 is connected to the scan line 705. The other of thesource and drain of the thin film transistor 701 is connected to thefirst power source line 103. The other of the source and drain of thethin film transistor 703 is connected to the gate of the thin filmtransistor 102. A gate of the thin film transistor 701 is connected tothe gate of the thin film transistor 102.

In FIGS. 7A and 7B, the thin film transistor 703 is provided on the pathbetween the gate of the thin film transistor 102 and one of the sourceand drain of the thin film transistor 701, however, it may be providedin another place such as on the path between the gate of the thin filmtransistor 701 and the second capacitor 108.

In the configurations shown in FIGS. 7A and 7B, the thin film transistor702 is selected to be turned on by a signal inputted to the scan line705, and the thin film transistor 703 is selected to be turned on by thepotential G4 inputted to the gate thereof, thereby a voltagecorresponding to the signal inputted to the signal line 704 is held inthe capacitor 108. In this manner, on/off of the thin film transistor102 and a value of a drain current thereof when turned on arecontrolled.

Operations of the pixels having the configurations shown in FIGS. 7A and7B are further described. The signal line 704 is inputted with a currenthaving a predetermined current value (hereinafter referred to as asignal current). When the thin film transistors 702 and 703 are selectedto be turned on, a signal current flows through the thin filmtransistors 702 and 703 to charge the capacitor 108. In this manner, avoltage (hereinafter referred to as a voltage corresponding to thesignal current) is held in the capacitor 108 so that the thin filmtransistor 701 supplies a drain current of the same amount as the signalcurrent. A potential difference between the gate and source of the thinfilm transistor 701 and a potential difference between the gate andsource of the thin film transistor 102 are equal. Provided that the thinfilm transistors 701 and 102 have the same polarity, approximately equalratios of channel width to channel length, and approximately the samecharacteristics, the thin film transistor 102 supplies a drain currentof approximately the same amount as the signal current. In this manner,a current supplied to the light emitting element 105 is controlled toperform a display.

After the thin film transistor 702 is selected to be turned off and asignal current is not inputted from the signal line to the pixel, avoltage corresponding to the signal current is held in the capacitor108. Accordingly, even after the signal current is not inputted from thesignal line to the pixel, the thin film transistor 102 supplies a draincurrent of approximately the same amount as the signal current. It is tobe noted that it is preferable that the thin film transistor 703 beselected to be turned off before or at the same time as the thin filmtransistor 702 is selected to be turned off. If the thin film transistor702 is selected to be turned off with the thin film transistor 703 beingon, a charge held in the capacitor 108 is discharged and a voltagecorresponding to the signal current cannot be held anymore.

The thin film transistors 702 and 703 can be selected to be turnedon/off at the same time. Accordingly, the thin film transistors 702 and703 can have the same polarity and the gate of the thin film transistor703 can be connected to the scan line 705. A wiring for inputting asignal to the gate of the thin film transistor 702 and a wiring forinputting a signal to the gate of the thin film transistor 703 can beshared, which can improve an aperture ratio of pixels.

In FIG. 7A, an operation of the circuit 106 for setting a potential ineach of the case where the thin film transistor 102 is selected to beturned on and the case where the thin film transistor 102 is selected tobe turned off is similar to Embodiment Mode 3. In FIG. 7B, an operationof the thin film transistor 107 in each of the case where the thin filmtransistor 102 is selected to be turned on and the case where the thinfilm transistor 102 is selected to be turned off is similar toEmbodiment Mode 4.

Embodiment 3 can be freely implemented in combination with embodimentmodes.

Embodiment 4

A specific example of a pixel configuration is described. FIG. 10 is across sectional diagram showing a pixel configuration of the invention.Reference numeral 1000 denotes a substrate, 1001 denotes a base film,1002 denotes a semiconductor layer, 1003 denotes a first insulatingfilm, 1004 denotes a gate electrode, 1005 denotes a second insulatingfilm, 1006 denotes an electrode, 1007 denotes a first electrode, 1008denotes a third insulating film, 1009 denotes a light emitting layer,and 1010 denotes a second electrode. Reference numeral 1100 denotes athin film transistor, 1011 denotes a light emitting element, and 1012denotes a capacitor.

The substrate 1000 may be formed of a glass substrate such as a bariumborosilicate glass and aluminoborosilicate glass, a quartz substrate, aceramic substrate and the like. Further, a metal substrate containingstainless steel or a semiconductor substrate each of which has aninsulating film over the surface may be used as well. A substrate formedof a flexible synthetic resin such as plastic may also be used. Asurface of the substrate 1000 may be planarized by polishing by a CMPmethod and the like.

The base film 1001 may be formed of an insulating film such as siliconoxide, silicon nitride or silicon nitride oxide. By providing the basefilm 1001, it can be prevented that an alkaline metal such as Na and analkaline earth metal in the substrate 1000 are dispersed into thesemiconductor layer 1002 and adversely affect characteristics of thethin film transistor 1100. In FIG. 10, the base film 1001 has a singlelayer structure; however, two layers or a plurality of layers more thantwo may be formed as well. It is to be noted that the base film 1001 isnot necessarily provided in the case where the dispersion of impuritiesis not a big problem, such as the case of using a quartz substrate.

The semiconductor layer 1002 may be formed of a patterned crystallinesemiconductor film or amorphous semiconductor film. The crystallinesemiconductor film can be obtained by crystallizing an amorphoussemiconductor film. A crystallizing method may be a laser crystallizingmethod, a thermal crystallizing method using RTA or an annealingfurnace, a thermal crystallizing method using a metal catalyst whichpromotes crystallization and the like. The semiconductor layer 1002 hasa channel forming region and a pair of impurity regions which are addedimpurity elements which impart conductivity. It is to be noted that animpurity region which is added impurity elements at a low concentrationmay be provided between the channel forming region and the pair ofimpurity regions.

The first insulating film 1003 can be formed of a single layer or aplurality of stacked layers, using silicon oxide, silicon nitride orsilicon nitride oxide and the like.

The gate electrode 1004 can be formed of a single layer structure or astacked-layer structure of an alloy or compound containing one or aplurality of elements selected from Ta, W, Ti, Mo, Al, Cu, Cr, and Nd.For example, stacked layers of TaN and W can be used as the gateelectrode 1004. A semiconductor film represented by a polycrystallinesilicon film which is added impurity elements which imparts conductivitymay be used as well.

The thin film transistor 1100 is formed of the semiconductor layer 1002,the gate electrode 1004, and the first insulating film 1003 between thesemiconductor layer 1002 and the gate electrode 1004. In FIG. 10, onlythe thin film transistor 1100 connected to the first electrode 1007 ofthe light emitting element 1011 is shown, however, a plurality of thinfilm transistors may be provided as well. Moreover, the thin filmtransistor 1100 is shown as a top gate transistor in this embodiment,however, a bottom gate transistor having a gate electrode beneath thesemiconductor layer or a dual gate transistor having gate electrodesabove and beneath the semiconductor layer may be employed as well.

The second insulating film 1005 may be formed of a single layer orstacked layers of an inorganic insulating film and an organic insulatingfilm. As the inorganic insulating film, a silicon oxide film formed by aCVD method, a SOG (Spin On Glass) method and the like can be used. Asthe organic insulating film, a film such as polyimide, polyamide, BCB(benzocyclobutene), acrylic or a positive photosensitive organic resin,and a negative photosensitive organic resin can be used.

Further, a material having a backbone structure of Si (silicon)-O(oxygen) bond can be used for the second insulating film 1005 as well.As a substituent for this material, an organic group (for example and analkyl group, aromatic carbon hydride) containing at least hydrogen isused. As the substituent, a fluoro group may be used as well. Moreover,a fluoro group and an organic group containing at least hydrogen mayalso be used.

The electrode 1006 may be formed of a single layer structure or astacked-layer structure of an alloy containing one or a plurality ofelements selected from Al, Ni, C, W, Mo, Ti, Pt, Cu, Ta, Au, and Mn. Forexample, a metal film Ti/Al/Ti formed by stacking Al and Ti can be usedas the electrode 1006. Further, an end portion of the electrode 1006formed over the second insulating film 1005 may be tapered, which canprevent a break of a film formed thereover.

One or both of the first electrode 1007 and the second electrode 1010can be a light transmissive electrode. As the light transmissiveelectrode, a light transmissive oxide conductive material such as indiumtin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), and zincoxide which is added gallium (GZO) can be used. Alternatively, ITO andITO containing silicon (hereinafter referred to as ITSO), ITO and ITOcontaining titanium oxide (hereinafter referred to as ITSO), ITO and ITOcontaining molybdenum oxide (hereinafter referred to as ITMO), no whichis added titanium, molybdenum, or gallium, and indium oxide containingsilicon oxide which is added 2 to 20% of zinc oxide (ZnO) may be used aswell.

The other of the first electrode 1007 and the second electrode 1010 maybe formed of a material which does not transmit light. For example, analkaline metal such as Li and Cs, an alkaline earth metal such as Mg,Ca, and Sr, an alloy containing these (Mg:Ag, Al:Li, Mg:In and the like)and a compound thereof (CaF₂ and calcium nitride), and a rare earthmetal such as Yb and Er can be used.

The third insulating film 1008 can be formed using a similar material tothat of the second insulating film 1005. The third insulating film 1008is formed in the periphery of the first electrode 1007 so as to cover anend portion of the first electrode 1007, and functions as a bank forseparating the light emitting layer 1009 between adjacent pixels.

The light emitting layer 1009 is formed of a single layer or a pluralityof layers. In the case where the light emitting layer 1009 is formed ofa plurality of layers, these layers can be categorized into a holeinjecting layer, a hole transporting layer, a light emitting layer, anelectron transporting layer, an electron injecting layer and the like inview of a carrier transporting property. It is to be noted that aboundary between each layer is not necessarily clear, but an interfacemay be unclear when materials forming each layer are partly mixed. Forthe each layer, an organic material and an inorganic material can beused. As the organic material, any one of a high molecular weight,medium molecular weight, and low molecular weight materials can be used.It is to be noted that the medium molecular weight material correspondsto an oligomer of which structure includes about 2 to 20 repetition(polymerization degree) of a single constitutional unit.

The light emitting element 1011 is formed of the light emitting layer1009, and the first electrode 1007 and the second electrode 1010 withthe light emitting layer 1009 interposed therebetween. One of the firstelectrode 1007 and the second electrode 1010 corresponds to an anodewhile the other corresponds to a cathode. The light emitting element1011 emits light when a forward bias voltage higher than a thresholdvoltage thereof is applied between the anode and cathode thereof and acurrent flows from the anode to the cathode.

The capacitor 1012 is formed of the third insulating film 1008, and thefirst electrode 1007 and the second electrode 1010 with the thirdinsulating film 1008 interposed therebetween. The capacitor 1012corresponds to the capacitor in the third configuration of theinvention, that is the capacitor 101 in embodiment modes and Embodiments1 to 3.

This embodiment can be freely implemented in combination with embodimentmodes and Embodiments 1 to 3.

Embodiment 5

A specific example of a pixel configuration which is different thanEmbodiment 4 is described. It is to be noted that the same portions asFIG. 10 are denoted by the same reference numerals and a descriptionthereon is omitted.

In a configuration of FIG. 11, the third insulating film 1008 of aportion overlapped with the first electrode 1007 is formed thin. Acapacitor 1112 is formed of the third insulating film 1008, and thefirst electrode 1007 and the second electrode 1010 with the thirdinsulating film 1008 interposed therebetween. The capacitor 1112corresponds to the capacitor in the third configuration of theinvention, that is the capacitor 101 in embodiment modes and Embodiments1 to 3. The capacitor 1112 requires a smaller area for an electrode toobtain the same capacitance compared to the capacitor 1012 of Embodiment4. In this manner, an aperture ratio of pixels can be improved.

This embodiment can be freely implemented in combination with embodimentmodes and Embodiments 1 to 3.

Embodiment 6

A specific example of a pixel configuration which is different thanEmbodiments 4 and 5 is described. Each of FIGS. 12A to 12C shows a crosssectional diagram showing a pixel configuration of the invention. It isto be noted that the same portions as FIG. 10 are denoted by the samereference numerals and a description thereon is omitted.

In the configuration of FIG. 12A, the capacitor 1212 is formed of thethird insulating film 1008, and the electrode 1006 and the secondelectrode 1010 with the third insulating film 1008 interposedtherebetween. The capacitor 1212 corresponds to the capacitor in thethird configuration of the invention, that is the capacitor 101 inembodiment modes and Embodiments 1 to 3.

As described in Embodiment 4, the electrode 1006 can be formed ofstacked layers. Each of FIGS. 12B and 12C shows an example of theelectrode 1006 having a stacked-layer structure. The electrode 1006 isformed of a first layer 1206 a, a second layer 1206 b, and a third layer1206 c. For example, Ti can be used for the first layer 1206 a, Al canbe used for the second layer 1206 b, and Ti can be used for the thirdlayer 1206 c.

In FIGS. 12B and 12C, portions of the second layer 1206 b and the thirdlayer 1206 c which are overlapped with the first layer 1206 a areremoved, and a portion of the first layer 1206 a only remains(hereinafter referred to as an extended portion of the first layer 1206a). A capacitor 1213 is formed of the third insulating film 1008, andthe extended portion of the first layer 1206 a and the second electrode1010 with the third insulating film 1008 interposed therebetween. Thecapacitor 1213 corresponds to the capacitor in the third configurationof the invention, that is the capacitor 101 in embodiment modes andEmbodiments 1 to 3.

As the capacitor 1213 is formed in the extended portion of the firstlayer 1206 a without the second layer 1206 b in the configurations ofFIGS. 12B and 12C, a defect such as a short-circuit between electrodescan be reduced even when the second layer 1206 b has poor planarity.Accordingly, the configurations shown in FIGS. 12B and 12C are effectivein particular in the case where a material having a relatively lowelectric resistance but poor planarity is used for the second layer 1206b and a material having a relatively high electric resistance but goodplanarity is used for the first layer 1206 a and the third layer 1206 c.For example, these configurations are effective in the case where Al isused for the second layer 1206 b and Ti is used for the first layer 1206a and the third layer 1206 c.

In FIG. 12C, the first layer 1206 a and the first electrode 1007 areconnected at the extended portion of the first layer 1206 a. Theelectrode 1006 of a portion overlapped with the first electrode 1007 isformed thin, which can prevent a break of the first electrode 1007 andensure a connection between the first electrode 1007 and the electrode1006.

This embodiment can be freely implemented in combination with embodimentmodes and Embodiments 1 to 5.

Embodiment 7

A specific example of a pixel having a different configuration thanEmbodiments 4 to 6 is described. FIG. 13 is a cross sectional diagramshowing a pixel configuration of the invention. It is to be noted thatthe same portions as FIGS. 10A to 12C are denoted by the same referencenumerals and a description thereon is omitted.

In the configuration of FIG. 13A, the third insulating film 1008 of aportion overlapped with the electrode 1006 (or the extended portion ofthe first layer 1206 a) is formed thin. A capacitor 1312 is formed ofthe third insulating film 1008, and the electrode 1006 and the secondelectrode 1010 with the third insulating film 1008 interposedtherebetween. The capacitor 1312 corresponds to the capacitor in thethird configuration, that is the capacitor 101 in embodiment modes andEmbodiments 1 to 3. The capacitor 1312 requires a smaller area for anelectrode to obtain the same capacitance, which can improve an apertureratio of pixel.

In FIGS. 13B and 13C, a capacitor 1313 is formed of the third insulatingfilm 1008, and the extended portion of the first layer 1206 a and thesecond electrode 1010 with the third insulating film 1008 interposedtherebetween. The capacitor 1313 corresponds to the capacitor in thethird configuration of the invention, that is the capacitor 101 inembodiment modes and Embodiments 1 to 3.

In FIG. 13C, the first layer 1206 a and the first electrode 1007 areconnected at the extended portion of the first layer 1206 a.

Effects of FIGS. 13B and 13C are similar to those of FIGS. 12B and 12Cof Embodiment 6. Further, the capacitor 1313 in FIGS. 13B and 13Crequires a smaller area for an electrode to obtain the same capacitanceas compared to the capacitor 1213 in FIGS. 12B and 12C. In this manner,an aperture ratio of pixel can be improved.

This embodiment can be freely implemented in combination with embodimentmodes and Embodiments 1 to 5.

Embodiment 8

Description is made with reference to FIGS. 14A to 14C on amanufacturing method of the third insulating film 1008 of which portionis formed thin described in Embodiments 5 and 7. In FIGS. 14A to 14C,the same portions as in FIG. 11 are denoted by the same referencenumerals and a description thereon is omitted.

In FIG. 14A, after forming the first electrode 1007, an insulating film1408 is formed. A photosensitive material is used for the insulatingfilm 1408. The insulating film 1408 is exposed by using a photo mask1400. The photo mask 1400 is provided with a first light transmissiveportion 1401, a second light transmissive portion 1402, and a lightshielding portion 1403. The first light transmissive portion 1401 may bean aperture. The intensity of light transmitting through the photo mask1400 is lower in the second light transmissive portion 1402 than in thefirst light transmissive portion 1401. The light shielding portion 1403does not transmit light almost at all. A half-tone mask as describedabove is used as the photo mask 1400.

In FIG. 14B, the insulating film 1408 is developed. The insulating film1408 of a portion overlapped with the light shielding portion 1403 isbarely etched. The insulating film 1408 of a portion exposed through thefirst light transmissive portion 1401 is largely etched. In this manner,an aperture portion 1411 in which a surface of the first electrode 1007is exposed is formed. The insulating film 1408 of a portion exposedthrough the second light transmissive portion 1402 is etched to someextent. In this manner, a thin portion 1412 is formed in the insulatingfilm 1408. In this manner, the insulating film 1408 having a thinportion is obtained. The insulating film 1408 in FIG. 14B corresponds tothe third insulating film 1008 described in Embodiments 5 and 7.

In FIG. 14C, the light emitting layer 1009 and the second electrode 1010are sequentially formed.

This embodiment can be freely implemented in combination with embodimentmodes and Embodiments 1 to 7.

Embodiment 9

Description is made with reference to FIGS. 15A to 15C on amanufacturing method of the third insulating film 1008 of which portionis formed thin described in Embodiments 5 and 7, which is different thanthe method described in Embodiment 8. In FIGS. 15A to 15C, the sameportions as FIG. 11 are denoted by the same reference numerals and adescription thereon is omitted.

In FIG. 15A, after forming the first electrode 1007, an insulating film1508 a is formed. An insulating film 1508 b is formed over theinsulating film 1508 a. A single layer or stacked layers of an inorganicinsulating film and an organic insulating film can be used as theinsulating films 1508 a and 1508 b. Further, a material having abackbone structure of a Si (silicon)-O (oxygen) bond can be used aswell. As a substituent for this material, an organic group (for example,an alkyl group and aromatic carbon hydride) containing at least hydrogenis used. As the substituent, a fluoro group may be used as well.Moreover, a fluoro group and an organic group containing at leasthydrogen may also be used.

The insulating film 1508 b is etched to form the first aperture portion1511 a and the second aperture portion 1512.

In FIG. 15B, the insulating film 1508 a is etched in the first apertureportion 1511 a to form a third aperture portion 1511 b. In this manner,the third aperture portion 1511 b in which a surface of the firstelectrode 1007 is exposed and a second aperture portion 1512 in which asurface of the insulating film 1508 a is exposed can be obtained. Thepatterned insulating films 1508 a and 1508 b correspond to the thirdinsulating film 1008 described in Embodiments 5 and 7. In this manner,the third insulating film 1008 having a thin portion is obtained.

In FIG. 15C, the light emitting layer 1009 and the second electrode 1010are sequentially formed.

The capacitor 1112 has capacitance with the insulating film 1508 a as adielectric substance; therefore, the insulating film 1508 a preferablyhas a high dielectric material such as a silicon nitride film, forexample.

This embodiment can be freely implemented in combination with embodimentmodes and Embodiments 1 to 7.

Embodiment 10

The display device and the driving method of the invention can beapplied to various electronic devices each having the display deviceincorporated in a display portion thereof.

The electronic devices include a camera (a video camera, a digitalcamera and the like), a projector, a head mounted display (a goggle typedisplay), a navigation system, a car stereo set, a personal computer, agame machine, a portable information terminal (a mobile computer, aportable phone, an electronic book or the like), an image reproducingdevice provided with a recording medium (specifically, a device whichreproduces a recording medium such as a DVD (Digital Versatile Disc) andhas a display which can display the reproduced image), and the like.Examples of the electronic devices are shown in FIGS. 9A to 9D.

FIG. 9A illustrates a notebook personal computer including a main body911, a housing 912, a display portion 913, a keyboard 914, an externalconnecting port 915, a pointing pad 916 and the like. The display deviceand driving method thereof of the invention are applied to the displayportion 913. By using the invention, a clear display of the displayportion 913 with a higher contrast can be realized with less powerconsumption. It is quite effective to apply the invention to thenotebook personal computer which requires a reduction in powerconsumption.

FIG. 9B illustrates an image reproducing device provided with arecording medium (specifically a DVD reproducing device), including amain body 921, a housing 922, a first display portion 923, a seconddisplay portion 924, a recording medium (a DVD and the like) readingportion 925, an operating key 926, a speaker portion 927 and the like.The first display portion 923 mainly displays image data while thesecond display portion 924 mainly displays text data. The display deviceand driving method thereof of the invention are applied to the firstdisplay portion 923 and the second display portion 924. By using theinvention, a clear display of the first display portion 923 and thesecond display portion 924 with a higher contrast can be realized withless power consumption.

FIG. 9C illustrates a portable phone including a main body 931, an audiooutput portion 932, an audio input portion 933, a display portion 934,operating switches 935, an antenna 936 and the like. The display deviceand driving method thereof of the invention are applied to the displayportion 934. By using the invention, a clear display of the displayportion 934 with a higher contrast can be realized with less powerconsumption. It is quite effective to apply the invention to theportable phone which requires a reduction in power consumption.

FIG. 9D illustrates a camera including a main body 941, a displayportion 942, a housing 943, an external connecting port 944, a remotecontrol receiving portion 945, an image receiving portion 946, a battery947, an audio input portion 948, operating keys 949 and the like. Thedisplay device and driving method of the invention are applied to thedisplay portion 942. By using the invention, a clear display of thedisplay portion 942 with a higher contrast can be realized with lesspower consumption.

This embodiment mode can be freely implemented in combination withembodiment modes and Embodiments 1 to 9.

This application is based on Japanese Patent Application serial no.2004-270477 filed in Japan Patent Office on 16th, Sep., 2004, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising a pixel, the pixelcomprising: a transistor comprising a semiconductor layer including achannel formation region, a gate, and a gate insulating layer interposedbetween the semiconductor layer and the gate; a first capacitor; asecond capacitor; a first insulating layer over the transistor; a secondinsulating layer over the first insulating layer, the second insulatinglayer comprising a first opening and a second opening, each of the firstopening and the second opening being formed in the pixel; and a lightemitting element comprising: a pixel electrode over the first insulatinglayer and below the second insulating layer, the pixel electrode beingelectrically connected to the semiconductor layer; a light emittinglayer over the pixel electrode; and a conductive layer over the secondinsulating layer and the light emitting layer, wherein the conductivelayer is located over the first opening and the second opening, whereinthe light emitting layer is located over the first opening while thelight emitting layer is not located over the second opening, wherein afirst electrode of the first capacitor is electrically connected to thegate and a second electrode of the first capacitor is directly connectedto the pixel electrode, and wherein the second capacitor and the lightemitting element are connected in parallel with each other.
 2. Thedisplay device according to claim 1, wherein the gate is located overthe semiconductor layer.
 3. The display device according to claim 1,wherein a first electrode of the second capacitor is the pixelelectrode, and wherein a second electrode of the second capacitor is theconductive layer.
 4. The display device according to claim 1, whereinthe pixel electrode is located below the first opening and the secondopening.
 5. The display device according to claim 1, further comprisinga means for setting a potential and a third insulating layer, whereinthe means is electrically connected to the conductive layer, and whereinthe third insulating layer is interposed between the first insulatinglayer and the second insulating layer and is located below the secondopening.
 6. The display device according to claim 1, further comprisinganother transistor, a first power source line, and a second power sourceline, wherein one of a source and a drain of the another transistor iselectrically connected to the pixel electrode and the other of thesource and the drain of the another transistor is directly connected tothe first power source line, wherein the second power source line iselectrically connected to the conductive layer, and wherein a potentialof the second power source line is lower than that of the first powersource line.
 7. A display device comprising a pixel, the pixelcomprising: a transistor comprising a semiconductor layer including achannel formation region, a gate, and a gate insulating layer interposedbetween the semiconductor layer and the gate; a first capacitor; asecond capacitor; a first insulating layer over the transistor; a firstconductive layer over the first insulating layer, the first conductivelayer overlapping with the channel formation region of the transistor; asecond insulating layer over the first insulating layer, the secondinsulating layer comprising a first opening and a second opening, eachof the first opening and the second opening being formed in the pixel;and a light emitting element comprising: a pixel electrode over thefirst insulating layer and below the second insulating layer, the pixelelectrode being electrically connected to the semiconductor layer; alight emitting layer over the pixel electrode; and a second conductivelayer over the second insulating layer and the light emitting layer,wherein the second conductive layer is located over the first openingand the second opening, wherein the light emitting layer is located overthe first opening while the light emitting layer is not located over thesecond opening, wherein a first electrode of the first capacitor iselectrically connected to the gate and a second electrode of the firstcapacitor is directly connected to the pixel electrode, and wherein thesecond capacitor and the light emitting element are connected inparallel with each other.
 8. The display device according to claim 7,wherein the gate is located over the semiconductor layer.
 9. The displaydevice according to claim 7, wherein a first electrode of the secondcapacitor is the pixel electrode, and wherein a second electrode of thesecond capacitor is the second conductive layer.
 10. The display deviceaccording to claim 7, wherein the pixel electrode is located below thefirst opening and the second opening.
 11. The display device accordingto claim 7, further comprising a means for setting a potential and athird insulating layer, wherein the means is electrically connected tothe second conductive layer, and wherein the third insulating layer isinterposed between the first insulating layer and the second insulatinglayer and is located below the second opening.
 12. The display deviceaccording to claim 7, further comprising another transistor, a firstpower source line, and a second power source line, wherein one of asource and a drain of the another transistor is electrically connectedto the pixel electrode and the other of the source and the drain of theanother transistor is directly connected to the first power source line,wherein the second power source line is electrically connected to thesecond conductive layer, and wherein a potential of the second powersource line is lower than that of the first power source line.
 13. Thedisplay device according to claim 7, wherein the pixel electrode isdirectly connected to the first conductive layer.
 14. The display deviceaccording to claim 7, wherein a third capacitor is formed using thefirst conductive layer, the second insulating layer, and the secondconductive layer.
 15. A display device comprising a pixel, the pixelcomprising: a first transistor; a second transistor comprising asemiconductor layer including a channel formation region, a gate, and agate insulating layer interposed between the semiconductor layer and thegate; a first capacitor; a second capacitor; a first insulating layerover the first transistor and the second transistor; a second insulatinglayer over the first insulating layer, the second insulating layercomprising a first opening and a second opening, each of the firstopening and the second opening being formed in the pixel; and a lightemitting element comprising: a pixel electrode over the first insulatinglayer and below the second insulating layer, the pixel electrode beingelectrically connected to the semiconductor layer; a light emittinglayer over the pixel electrode; and a conductive layer over the secondinsulating layer and the light emitting layer, wherein the conductivelayer is located over the first opening and the second opening, whereinthe light emitting layer is located over the first opening while thelight emitting layer is not located over the second opening, wherein afirst electrode of the first capacitor is electrically connected to thegate and a second electrode of the first capacitor is directly connectedto the pixel electrode, wherein the second capacitor and the lightemitting element are connected in parallel with each other, and whereinone of a source and a drain of the second transistor is electricallyconnected to the gate.
 16. The display device according to claim 15,wherein the gate is located over the semiconductor layer.
 17. Thedisplay device according to claim 15, wherein a first electrode of thesecond capacitor is the pixel electrode, and wherein a second electrodeof the second capacitor is the conductive layer.
 18. The display deviceaccording to claim 15, wherein the pixel electrode is located below thefirst opening and the second opening.
 19. The display device accordingto claim 15, further comprising a means for setting a potential and athird insulating layer, wherein the means is electrically connected tothe conductive layer, and wherein the third insulating layer isinterposed between the first insulating layer and the second insulatinglayer and is located below the second opening.
 20. The display deviceaccording to claim 15, further comprising a third transistor, a firstpower source line, and a second power source line, wherein one of asource and a drain of the third transistor is electrically connected tothe pixel electrode and the other of the source and the drain of thethird transistor is directly connected to the first power source line,wherein the second power source line is electrically connected to theconductive layer, and wherein a potential of the second power sourceline is lower than that of the first power source line.
 21. A displaydevice comprising a pixel, the pixel comprising: a first transistor: asecond transistor comprising a semiconductor layer including a channelformation region, a gate, and a gate insulating layer interposed betweenthe semiconductor layer and the gate; a first capacitor; a secondcapacitor; a first insulating layer over the first transistor and thesecond transistor; a first conductive layer over the first insulatinglayer, the first conductive layer overlapping with the channel formationregion of the second transistor; a second insulating layer over thefirst insulating layer, the second insulating layer comprising a firstopening and a second opening, each of the first opening and the secondopening being formed in the pixel; and a light emitting elementcomprising: a pixel electrode over the first insulating layer and belowthe second insulating layer, the pixel electrode being electricallyconnected to the semiconductor layer; a light emitting layer over thepixel electrode; and a second conductive layer over the secondinsulating layer and the light emitting layer, wherein the secondconductive layer is located over the first opening and the secondopening, wherein the light emitting layer is located over the firstopening while the light emitting layer is not located over the secondopening, wherein a first electrode of the first capacitor iselectrically connected to the gate and a second electrode of the firstcapacitor is directly connected to the pixel electrode, wherein thesecond capacitor and the light emitting element are connected inparallel with each other, and wherein one of a source and a drain of thesecond transistor is electrically connected to the gate.
 22. The displaydevice according to claim 21, wherein the gate is located over thesemiconductor layer.
 23. The display device according to claim 21,wherein a first electrode of the second capacitor is the pixelelectrode, and wherein a second electrode of the second capacitor is thesecond conductive layer.
 24. The display device according to claim 21,wherein the pixel electrode is located below the first opening and thesecond opening.
 25. The display device according to claim 21, furthercomprising a means for setting a potential and a third insulating layer,wherein the means is electrically connected to the second conductivelayer, and wherein the third insulating layer is interposed between thefirst insulating layer and the second insulating layer and is locatedbelow the second opening.
 26. The display device according to claim 21,further comprising a third transistor, a first power source line, and asecond power source line, wherein one of a source and a drain of thethird transistor is electrically connected to the pixel electrode andthe other of the source and the drain of the third transistor isdirectly connected to the first power source line, wherein the secondpower source line is electrically connected to the second conductivelayer, and wherein a potential of the second power source line is lowerthan that of the first power source line.
 27. The display deviceaccording to claim 21, wherein the pixel electrode is electricallyconnected to the semiconductor layer through the first conductive layer.28. The display device according to claim 21, wherein a third capacitoris formed using the first conductive layer, the second insulating layer,and the second conductive layer.